How to bevel edges next to ngons?
You need to have an inner edge loop on the top side of the edge in order for the bevel tool to work properly. 

You either need to:


Select all the vertical side faces and extrude outward along their normals 


OR 


Delete some of the top faces that connect to the corner
edge. Then you can extrude the corner edge in slightly to achieve
the inner edge loop.